1. Field of the Invention
The present invention relates to a power steering apparatus suitable for use in automobiles and the like, and more particularly, it relates to the improvement of a hydraulic power steering apparatus which is designed to enhance rigidity of a steering operation, and to obtain a steady feeling of the steering operation at and around a neutral position of a steering wheel.
2. Description of the Prior Art
Power steering apparatuses having a control valve comprising open control valve portions and closed control valve portions have been proposed in order to give a driver a steady steering feeling in the vicinity of a neutral position of a steering wheel. An example of them is shown in U.S. Pat. No. 5,515,938 of Kaga et al.
In the power steering apparatus, as shown in FIG. 1, a control valve 3 contains two kinds of control valve portions, one is an open control valve portion 1 and the other is a closed control valve portion 2. In a vicinity of a neutral position of a steering wheel, since overlap variable orifices v5, v6 in the closed control valve portion 2 are substantially closed in order to prevent supply of pressurized fluid for a power cylinder 4, the pressure in the power cylinder 4 is maintained to be extremely low or substantially zero even if a pressure of a pump 5 supplied to the control valve 3 rises in accordance with a slight rotation of the steering wheel in the vicinity of the neutral position. Therefore, the power steering apparatus is capable of giving a driver the steady steering feeling in the vicinity of the neutral position of the steering wheel.
In the power steering apparatus, two pairs of the open control valve portion 1 and the closed control valve portion 2 are arranged alternately around a cylindrical body of the control valve 3. On condition that the steering wheel is not or slightly rotated in the vicinity of the neutral position, each of the open control valve portions 1 mainly controls the pressurized fluid supplied from the pump 5. On condition that the steering wheel is rotated, both the open and the closed control valve portions control it.
In the vicinity of the neutral position, the open control valve portion 1 is designed to control the pressurized fluid through both sides of its orifice portion which form unlapped variable orifices v1, v3 (or v2, v4). However, the closed control valve portion 2 is designed to control the pressurized fluid through only one side of its orifice portion which forms an unlapped variable orifice v7 (or v8) because the other side forms the overlap variable orifice v5 (or v6). Since the closed control valve portion 2 thus decreases the route of the pressurized fluid, a configuration of the power steering apparatus with the closed control valve portion 2 works against prevention of noise caused by the pressurized fluid passing through the control valve 3, as compared with one of a power steering apparatus without the closed control valve portion 2. Therefore, there is a need for the power steering apparatus with the closed control valve portion 2 to exclude the additional influence of noise except for the configurative influence thereof.
A way to prevent the noise is the above-mentioned configuration that two pairs of the open and the closed control valve portions are arranged alternately on the control valve 3. The open control valve portions 1 diametrically face each other on a circuit of the control valve 3. The closed control valve portions 2 also face each other there. Since this configuration avoids a large flow of the pressurized fluid that concentrates in one route in the control valve 3, it is capable of preventing the concentrated pressurized fluid from enlarging the noise of the control valve 3. However, the applicant has found another negative collateral influence described hereinafter in the construction of the prior art.
As shown in FIG. 2 and FIG. 3, the closed control valve portion 2 consists of the overlap variable orifices v5, v6 on a passage connected to the pump 5 of FIG. 1 and the unlapped variable orifices v7, v8 on passages connected to the power cylinder 4 and a reservoir 6 in FIG. 1. In the configuration, it is ideal for a characteristic of the control valve 3 at the neutral position that an amount .theta.1 of an overlapping portion of each overlap variable orifice v6(v5) in FIG. 2(A) equals an amount .theta.2 of an unlapping portion of each unlapped variable orifice v8(v7) in FIG. 3(A). Therefore, it is designed that the overlap variable orifice v6(v5) finishes overlapping to open itself at the same time when the unlapped variable orifice v8(v7) finishes unlapping to close itself. Chambers 2A1 and 2B1 in FIGS. 2, 3 are formed on every edge of the variable orifices v5 to v8 to lead the pressurized fluid. Here, the variable orifice with a parenthesis () shows one condition that the steering wheel is rotated in an opposite direction in FIG. 2 and FIG. 3.
However, in practical manufacture, since it is almost impossible work to produce the variable orifices v5 to v8 with accurate width in accordance with engineering dimensions of a drawing thereof, it is unavoidable to cause dispersion of the width of the variable orifices v5 to v8. Accordingly, when the overlap variable orifice v6(v5) finishes overlapping, as shown in FIG. 2(B), the unlapped variable orifice v8(v7) does not yet finish unlapping because of dispersion .epsilon.1, as shown in FIG. 3(B), or it already starts overlapping because of dispersion .epsilon.2, as shown in FIG. 3(C).
Therefore, the dispersion brings about a difference between opening areas of two closed control valve portions 2 which diametrically face each other on the circuit of the control valve 3. It causes also a difference between flows of a pair of the closed control valve portions 2. As a result, there is a problem that the unbalance of the flows will enlarge the noise of the control valve 3 when the steering wheel is rotated.